Devices and methods for addressing optical edge effects in connection with etched trenches

ABSTRACT

In a first aspect of the invention, a modified semiconductor substrate is provided. The modified substrate comprises: (1) a semiconductor substrate; (2) at least one buffer layer provided over at least a portion of the substrate; and (3) a plurality of trenches comprising (a) a plurality of internal trenches that extend into the semiconductor substrate and (b) at least one shallow peripheral trench that extends into the at least one buffer layer but does not extend into the semiconductor substrate. In another aspect, a method of selectively providing trenches in a semiconductor substrate is provided. According to a further aspect of the invention, a trench DMOS transistor structure that includes at least one peripheral trench and a plurality of internal trenches is provided. The structure comprises: (1) a substrate of a first conductivity type; (2) a body region on the substrate having a second conductivity type, wherein the peripheral and internal trenches extend through the body region; (3) an insulating layer that lines each of the peripheral and internal trenches; (4) a first conductive electrode overlying each insulating layer; and (5) source regions of the first conductivity type in the body region adjacent to the each internal trench, but not adjacent to the at least one peripheral trench.

At present, semiconductor process technology is capable of creatingfeatures having dimensions well into the submicron range. At this levelof miniaturization, feature size variations due to what is commonlyreferred to as the “optical proximity effect” can become significant. Ingeneral, proximity effects are variations in feature dimensions that aredue to the proximity of other nearby features. In particular, opticalproximity effects are proximity effects that occur during opticallithography. As a result of optical proximity effects, the size of agiven feature can vary based on its spacing from other features.

Among the phenomena contributing to optical proximity effects arediffraction patterns associated with imaged features. One example of anoptical proximity effect is the difference in dimension that can occurbetween an isolated printed line and a printed line in a dense array ofequal lines.

Specific consequences of optical proximity effects include situationswhere internal features, which are surrounded by other features, andperipheral features, which are not, differ substantially. (Under thesecircumstances, optical proximity effects are frequently referred to asoptical edge effects.) For example, at present, during photolithographicprocesses at submicron feature sizes, peripheral photoresist featuresfrequently display a significant optical edge effect. As a result,etched silicon trenches, among other features, are frequently andadversely affected. Accordingly, devices employing etched silicontrenches, such as trench DMOSFETS (double diffused metal oxidesemiconductor field effect transistors), trench Schottky barrierrectifiers, DRAM (dynamic random access memory) devices, and devices inwhich trenches are used to isolate separate integrated circuits, arelikewise frequently and adversely effected by the optical edge effect.

An example of such an edge effect is presented in FIG 1A and 1B. Thesefigures illustrate a situation where trenches are etched using aperturesbetween the photoresist features. More specifically, as seen in FIG 1A,a silicon substrate 10 is provided with photoresist features 15 a, 15 b,15 c, 15 d via an optical lithography process. As shown in this figure,the internal features 15 a, 15 b and 15 c, each of which is positionedbetween other features (the feature to the left of internal feature 15 ais not shown here), have substantially vertical sidewalls.Unfortunately, as a consequence of the optical edge effect discussedherein, peripheral feature 15 d, which is not positioned between otherfeatures, has a substantially oblique sidewall as shown.

FIG. 1B illustrates the results that are obtained after subjecting thephotoresist-patterned silicon substrate to an etch step. As can be seenin this figure, due to the substantially vertical nature of thesidewalls associated with photoresist features 15 a, 15 b and 15 c,silicon sidewalls 10 a, 10 b and 10 c are also substantially vertical.In contrast, due to the substantially oblique nature of the sidewallsassociated with photoresist feature 15 d, silicon sidewall 10 d is alsosubstantially oblique, resulting in a sharp corner at the trench bottom.

In other instances, a silicon substrate is etched using a silicon oxideor silicon nitride photomask. Referring to FIG. 2A, a silicon oxide ornitride layer is etched via photoresist features 15 a, 15 b, 15 c, 15 d,to form silicon oxide or silicon nitride features 17 a, 17 b, 17 c, 17 don silicon substrate 10. As shown in this figure, the internalphotoresist features 15 a, 15 b, 15 c, each of which is positionedbetween other photoresist features, have substantially verticalsidewalls, while the peripheral photoresist feature 15 d, which is notpositioned between other photoresist features, has a substantiallyoblique sidewall. The same is true of the silicon oxide or nitridefeatures 17 a-17 d. Photoresist features 15 a, 15 b, 15 c and 15 d arethen removed, leaving only oxide or nitride features 17 a, 17 b, 17 cand 17 d. FIG. 2B illustrates the result of etching the siliconsubstrate 10 using silicon oxide or silicon nitride features 17 a, 17 b,17 c and 17 d alone as masking features. As can be seen, the results arelargely the same as those achieved when the substrate 10 is etched usingphotoresist features 15 a, 15 b, 15 c and 15 d (see FIG. 1B).Specifically, due to the substantially vertical nature of the sidewallsassociated with silicon oxide or silicon nitride features 17 a, 17 b, 17c, silicon sidewalls 10 a, 10 b and 10 c are also substantiallyvertical. Furthermore, silicon oxide or silicon nitride feature 17 d hasa substantially oblique sidewall, which results in a trench featurehaving a substantially oblique silicon sidewall 10 d and an accompanyingsharp corner at the trench bottom.

In still other instances, a silicon substrate is etched through a maskdefined by both photoresist features and silicon oxide or nitridefeatures. As shown in FIG. 3, the internal photoresist features 15 a, 15b, 15 c, each of which is positioned between other photoresist features,have substantially vertical sidewalls, while the peripheral photoresistfeature 15 d, which is not positioned between other photoresistfeatures, has a substantially oblique sidewall as shown. The same istrue of the silicon oxide or nitride features 17 a-17 d. As to thesilicon substrate 10, due to the substantially vertical nature of thesidewalls associated with features 15 a/17 a, 15 b/17 b and 15 c/17 c,silicon sidewalls 10 a, 10 b and 10 c are also substantially vertical.In contrast, due to fact that feature 15 d/l7 d is oblique and iscomprised of a combination of photoresist and oxide or nitride, a sharpcorner is formed at the trench bottom, as was observed in connectionwith FIGS. 1 and 2B. Moreover, the silicon substrate 10 is undercut atthe interface that is formed with the oxide or nitride feature 17 d.

In each of the above cases, the optical proximity effect producesundesirable trench characteristics, including sloping sidewalls andsharp-cornered bottoms. Accordingly, there is a need in the art toaddress optical proximity effects on etched trench features.

Others have addressed problems arising from optical proximity effects inDRAM applications by putting dummy trenches around the cells. See, e.g.,J. Fung Chen, Tom Laidig, Kurt E. Wampler and Roger Caldwell, “PracticalMethod for Full-Chip Optical Proximity Correction,” SPIE Proceedings,Vol.3051,1997; J. Fung Chen, Tom Laidig, Kurt E. Wampler and RogerCaldwell, “An OPC Roadmap to 0.14 mm Design Rules,” paper presented atBACUS, 1997; J. Li, D. Bernard, J. Rey, V. Boksha, “Model-Based OpticalProximity Correction Including Photo-resist Effects,” Proc. SPIE,V.3051, 1997, P.643-651; N. Shamma, F. Sporon-Fiedler, E. Lin, “A Methodfor Correction of Proximity Effect in Optical Lithography,” KTIMicrolithography Seminar Interface '91, P.145; Chris A. Mack,“Evaluating Proximity Effects Using 3-D Optical Lithography Simulation,”Semiconductor International July 1996 P.237; O. Otto etc, “Automatedoptical proximity correction—a rule-based approach,” SPIE Proceedings,V.2197,P.278, 1994; A. Kornblit etc, “Role of etch pattern fidelity inthe printing of optical proximity corrected photomasks,” EIPB'95, 1995.

However, a need nonetheless remains in the art for alternative methodsof addressing these problems.

SUMMARY OF THE INVENTION

These and other needs in the art are addressed by the present invention.

According to a first aspect of the present invention, a modifiedsemiconductor substrate is provided. The modified substrate comprises:(1) a semiconductor substrate; (2) at least one buffer layer providedover at least a portion of the substrate; and (3) a plurality oftrenches comprising (a) a plurality of internal trenches that extendinto the semiconductor substrate and (b) at least one shallow peripheraltrench that extends into the at least one buffer layer but does notextend into the semiconductor substrate.

In one preferred embodiment, the at least one buffer layer is providedover the semiconductor substrate in the area of the at least one shallowperipheral trench, while no buffer layer is provided over thesemiconductor substrate in the area of the plurality of internaltrenches.

In another preferred embodiment, at least one buffer layer is providedover the semiconductor substrate in the area of the at least one shallowperipheral trench and at least one buffer layer is provided over thesemiconductor substrate in the area of the plurality of internaltrenches. However, the at least one buffer layer in the area of theplurality of internal trenches is thinner than the at least one bufferlayer in the area of the at least one shallow peripheral trench. (Forexample, the at least one buffer layer in the area of the plurality ofinternal trenches can consist of a single buffer layer, while the atleast one buffer layer in the area of the at least one shallowperipheral trench can consist of two buffer layers.) As a result, eachinternal trench extends through the at least one buffer layer in thearea of the plurality of internal trenches and into the semiconductorsubstrate, while each shallow peripheral trench does not extend throughthe at least one buffer layer in the area of the at least one shallowperipheral trench (and thus does not extend into the semiconductorsubstrate).

According to another aspect of the present invention, a method ofproviding trenches in a semiconductor substrate is provided. The methodcomprises (1) providing a semiconductor substrate; (2) providing apatterned etch resistant layer over the substrate, the patterned layerhaving a plurality of trench apertures comprising (a) at least oneperipheral trench aperture and (b) a plurality of internal trenchapertures; (3) providing at least one buffer layer between eachperipheral trench aperture and the semiconductor substrate; and (4)conducting an etching process, wherein an internal trench is etched inthe semiconductor substrate at each internal trench aperture position,and a peripheral trench is prevented from being etched into thesemiconductor substrate at each peripheral aperture position by the atleast one buffer layer.

In one preferred embodiment, the method further comprises providing atleast one buffer layer between each internal trench aperture and thesemiconductor substrate. However, the at least one buffer layer betweeneach peripheral trench aperture and the semiconductor substrate has anaggregate thickness that is greater than the at least one buffer layerbetween each internal trench aperture and the semiconductor substrate.(For example, the at least one buffer layer between each internal trenchaperture and the semiconductor substrate can consist of a single bufferlayer, while the at least one buffer layer between each peripheraltrench aperture and the semiconductor substrate consists of two bufferlayers.) As a result, an internal trench is etched through the at leastone buffer layer and into the semiconductor substrate at each internaltrench aperture position during the etching procedure, while a trench isnot etched through the at least one buffer layer (and hence not into thesubstrate) at each peripheral trench aperture position.

Preferred buffer layers include oxide layers and nitride layers. Apreferred substrate is a silicon substrate.

A number of devices can be used in connection with the modifiedsubstrate and method of the present invention, including trench DMOStransistors, trench Schottky barrier rectifiers, and a DRAM device.

According to a further aspect of the present invention, a trench DMOStransistor structure that includes at least one peripheral trench and aplurality of internal trenches is provided. The structure comprises: (1)a substrate of a first conductivity type; (2) a body region on thesubstrate having a second conductivity type, wherein the peripheral andinternal trenches extend through the body region; (3) an insulatinglayer that lines each of the peripheral and internal trenches; (4) afirst conductive electrode overlying each insulating layer; and (5)source regions of the first conductivity type in the body regionadjacent to the each internal trench, but not adjacent to the at leastone peripheral trench. The structure can also comprise a drain electrodedisposed on a surface of the substrate opposing the body region and asource electrode disposed over at least a portion of the source regions.

Preferably, the first conductivity type is n-type conductivity and thesecond conductivity type is p-type conductivity. Preferably, theinsulating layer is an oxide layer and the conductive electrodecomprises polysilicon. In certain preferred embodiments, the trench DMOStransistor structure further comprises an insulating region (such as aborophosphosilicate glass structure) over each first conductiveelectrode in the internal trenches.

One advantage of the present invention is that adverse optical edgeeffects associated with peripheral trench features are dealt with in aneffective and economical manner.

Another advantage of the present invention is that the performance ofproducts with peripheral trench features, such as trench DMOS devices,trench Schottky battier rectifiers, DRAM devices, and other devicesemploying peripheral trench features, is substantially improved.

These and other embodiments and advantages of the present invention willbecome readily apparent upon review of the Detailed Description andClaims to follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a semiconductor substrate afterapplication of a patterned photoresist pattern.

FIG. 1B is a cross-sectional view of a semiconductor substrate afterapplication of a patterned photoresist pattern and subsequent etching.

FIG. 2A is a cross-sectional view of a semiconductor substrate that hasbeen provided with photoresist and oxide or nitride features.

FIG. 2B is a cross-sectional view of the semiconductor substrate of FIG.2A, after removal of photoresist and after etching through the remainingoxide or nitride features.

FIG. 3 is a cross-sectional view of the semiconductor substrate of FIG.2A, after etching through photoresist and oxide or nitride features.

FIGS. 4A-4C are cross-sectional views illustrating a process of trenchformation according to an embodiment of the present invention.

FIGS. 5A-5C are cross-sectional views illustrating a process of trenchformation according to an embodiment of the present invention.

FIG. 6 is a cross-sectional view of a trench DMOS transistor accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied indifferent forms and should not be construed as being limited to theembodiments set forth herein.

As used herein, the term “peripheral trench” refers to a trench, or aportion thereof, which is formed in a surface and is flanked on oneside, but not the other, by one or more similar structures. Similarly,the term “internal trench” refers to a trench, or a portion thereof,which is formed in a surface and is flanked on both sides by one or moresimilar structures. An “internal trench aperture” refers to an aperturein a patterned etch resistant layer which, upon a sufficient depth ofetching through the aperture, leads to the formation of an internaltrench. A “peripheral trench aperture” refers to an aperture in apatterned etch resistant layer which, upon a sufficient depth of etchingthrough the aperture, leads to the formation of a peripheral trench. Asseen below, in some embodiments of the present invention, a peripheraltrench aperture does not actually lead to the formation of a trench in asemiconductor substrate upon etching, but rather leads instead to onlythe formation of a shallow trench in a buffer layer.

A first embodiment of the present invention is presented in accordancewith FIGS. 4A-4C. As shown in FIG. 4A, a nitride or oxide feature 102,preferably formed from a silicon oxide or silicon nitride layer, isformed using techniques known in the art, for example chemical vapordeposition (CVD), on surface 100 a of substrate 100, preferably asilicon substrate.

Then, as seen in FIG. 4B, a patterned photoresist layer with features104 a, 104 b, 104 c and 104 d is provided on portions of surface 100 aand on portions of oxide or nitride feature 102 (features 104 c and 104d partially cover opposing sides 102 a and 102 b of feature 102, leavingthe central top surface of feature 102 exposed). Unlike features 104 a,104 b and 104 c, feature 104 d is not flanked by two other features andis hence suffers from an optical edge effect, as evidenced in FIG. 4B byoblique face 104 o.

The structure of FIG. 4B is then subjected to an etch step, such as areactive ion etching (RIE) step, wherein substrate 100 is preferentiallyetched relative to oxide or nitride feature 102. As a result, as seen inFIG. 4C, trenches 106 are formed between features 104 a and 104 b, aswell as between features 104 b and 104 c, in substrate 100. Due to thepresence of oxide or nitride feature 102 between features 104 c and 104d, however, only a shallow trench 107 is formed in nitride feature 102,and no trench is formed in substrate 100

As seen from FIG. 1B above, had a trench been formed in the substrate atthis peripheral position in the absence of oxide or nitride feature 102,such a trench would have been expected to have a sharp-cornered bottom,due to the optical edge effect. Hence, by providing an oxide or nitridefeature 102 underneath a peripheral trench aperture in photoresist layer104, an adverse outcome is prevented.

A second embodiment is presented in connection with FIGS. 5A-5C. As seenin FIG. 5A, a nitride or oxide feature 102 is formed using techniquesknown in the art, such as CVD, on surface 100 a of substrate 100.Subsequently, a nitride or oxide layer 103 is provided over surface 100a and over oxide or nitride feature 102, also using techniques known inthe art such as CVD.

As seen in FIG. 5B, a patterned photoresist layer, having photoresistfeatures 104 a, 104 b, 104 c, 104 d, is provided over oxide or nitridelayer 103. Then, an oxide or nitride etching process, such as reactiveion etching, is carried out in which oxide or nitride layer 103 ispatterned, and trenches 106 are formed in the substrate 100, as shown.This etching step is sufficient to etch through oxide or nitride layer103, but it is not sufficient to also etch through oxide or nitridelayer 102. Hence, apertures are provided in the oxide or nitride layer103 between photoresist features 104 a and 104 b, between features 104 band 104 c, as well as between features 104 c and 104 d. Moreover,trenches 106 are etched in substrate 100 through apertures defined byfeatures 104 a/103 a and 104 b/103 b, as well as by features 104 b/103 band 104 c/103 c. However, due to the additional oxide or nitridethickness, an aperture is not provided in oxide or nitride layer 102between photoresist features 104 c and 104 d, so no trench is formed insubstrate 100 at the location. As seen from FIG. 3 above, had a trenchbeen formed in the substrate 100 at a peripheral position defined byfeatures 104 c/013 c and 104 d/103 d, such a trench would have beenexpected to have both a sharp-cornered bottom and an undercut at theright-hand side of the trench.

The structure shown in FIG. 5C is formed by the same procedure discussedabove in connection with FIGS. 5A and 5B, except that photoresistfeatures 104 a, 104 b, 104 c and 104 d are removed prior to the trenchetch step. Had a trench been formed in the substrate 100 at a peripheralposition defined by features 103 c and 103 d, such a trench would havebeen expected to have suffered from an adverse optical edge effect likethat shown in FIG. 2B, wherein the trench has a sloping sidewall and asharp-cornered bottom.

A further embodiment of the invention is set forth in connection withFIG. 6. FIG. 6 illustrates two trench DMOS cells 250 constructed inaccordance with the present invention. Like prior art devices, trenchDMOS cells 250 include, within this embodiment, an n+ substrate 200 uponwhich is grown a lightly n-doped epitaxial layer 202, which serves asthe drain for the DMOS cells 250. A conductive layer (not shown) isapplied to the bottom of n+ substrate and acts as a common drain contactfor the DMOS cells. Within portions of n-doped epitaxial layer 202, bodyregions 204 of opposite (p-type) conductivity are provided and act asthe gate region for the DMOS cells 250. n+ regions 212 are alsoprovided, which act as sources for the DMOS cells 250. Conductive layer216 acts as a common source contact for the DMOS cells 250, shortingsources (i.e., n+ regions 212) with one another. Trench regions linedwith oxide layers 206 a and filled with polysilicon 210 a are provided.The filled trenches filled with oxide 206 a and polysilicon 210 a act asgate electrodes for the DMOS cells 250. Polysilicon 210 a is insulatedfrom conductive layer 216 (source contact) by BPSG (borophosphosilicateglass) structures 214, allowing the gates and sources to beindependently biased.

As noted above, due to optical edge effects, peripheral trenches aretypically defective. As a result, in the instance where a peripheralDMOS device is formed, shorting between the gate and source of theperipheral DMOS device frequently occurs. To prevent this fromhappening, a dummy peripheral device 252 is created in this embodiment.In this case, although the peripheral (right-hand) trench is filled withoxide layer 206 b and polysilicon 210 b, it is not provided with an n+source region (and hence need not be provided with a BPSG insulatingstructure). In this way, the possibility that shorting between gate andsource will occur in connection with the peripheral trench is prevented,because a source is entirely eliminated.

As previously noted, polysilicon regions 210 a associated with DMOScells 250 must be electrically insulated from source contact 216 forproper operation. In contrast, the polysilicon region 210 b associatedwith dummy peripheral device 252 can be shorted to source contact 216.Frequently, polysilicon regions 210 a associated with DMOS cells 250 areelectrically connected together (not shown), allowing a group ofdiscrete devices to behave as if it were a single large transistor.However since source contact 216 is shorted to polysilicon region 210 bin dummy peripheral device 252, care must be taken to ensure thatpolysilicon 210 b is not electrically connected to polysilicon 210 a.Otherwise a short would be established between the sources and gates ofthe DMOS device(s). This would typically be accomplished by isolatingthe peripheral trench from the other trenches.

A process for forming the structure of FIG. 6 is now briefly described.During the initial process steps, the DMOS cells 250 and the dummyperipheral device 252 are constructed in the same way, using processsteps that are well known in the art. For example, an N-doped epitaxiallayer 202 is grown on a conventionally N+ doped substrate 200. Then aP-body region 204 is formed in an implantation and diffusion step. Sincethe P-body region is uniform across the substrate, no mask is needed.The surface of the epitaxial layer is then covered with an oxide layer,which is conventionally exposed and patterned to leave mask openings inthe oxide layer. Trenches are dry etched through the mask openings, forexample, by reactive ion etching. As seen in FIG. 6, the peripheral(right-hand) trench corresponding to dummy peripheral device 252 istypically flawed due to the optical edge effect. An oxide layer 206 isthen deposited on the entire structure so that it covers the trenchwalls and the surface of P-body region 204. Next, a polysilicon layer210 is provided over the entire surface, filling the trenches. Thepolysilicon layer 210 is typically doped with phosphorous chloride orimplanted with arsenic or phosphorous to reduce its resistivity.

At this point, a masking layer is applied over the peripheral(right-hand) trench. Then, polysilicon layer 210 and oxide layer 206 areetched to optimize the thickness of the polysilicon layer 210 and exposeportions of the P-body between the trenches. Next, a photoresist maskingprocess is used to form a patterned masking layer having apertures thatdefine n+ regions 212. The n+ regions 212 are typically formed in animplantation and diffusion process. No aperture is provided, and henceno N+ region is formed, in connection with the formation of dummyperipheral device 252. Masking layers are then removed in a conventionalmanner. Subsequently, the DMOS cells are completed in a conventionalmanner by forming and patterning a BPSG layer over the structure todefine BPSG regions 214. (As previously noted, since dummy peripheraldevice contains no N+ source region, no BPSG region is needed.)Conductive layer 216 is then applied over the entire structure as shown.

Although various embodiments are specifically illustrated and describedherein, it will be appreciated that modifications and variations of thepresent invention are covered by the above teachings and are within thepurview of the appended claims without departing from the spirit andintended scope of the invention. As a specific example, the method ofthe present invention may be used to form a structure in which theconductivities of the various semiconductor regions are reversed fromthose described herein.

What is claimed is:
 1. A modified semiconductor substrate comprising:semiconductor substrate; at least one buffer layer provided over atleast a portion of said substrate; and a plurality of trenchescomprising (a) a plurality of internal trenches that extend into saidsemiconductor substrate and (b) at least one shallow peripheral trenchthat extends into said at least one buffer layer but does not extendinto the semiconductor substrate, wherein said at least one buffer layeris provided over said semiconductor substrate in the area of said atleast one shallow peripheral trench and no buffer layer is provided oversaid semiconductor substrate in the area of said plurality of internaltrenches.
 2. The modified semiconductor substrate of claim 1, whereinthe at least one buffer layer is an oxide layer.
 3. The modifiedsemiconductor substrate of claim 1, wherein the semiconductor substrateis a silicon substrate.
 4. A trench DMOS transistor comprising themodified substrate of claim
 1. 5. A DRAM device comprising the modifiedsubstrate of claim
 1. 6. A modified semiconductor substrate comprising:a semiconductor substrate; at least one buffer layer provided over atleast a portion of said substrate; and a plurality of trenchescomprising (a) a plurality of internal trenches that extend into saidsemiconductor substrate and (b) at least one shallow peripheral trenchthat extends into said at least one buffer layer but does not extendinto the semiconductor substrate, wherein the at least one buffer layeris a nitride layer.
 7. A trench Schottky barrier rectifier comprising amodified semiconductor substrate, the substrate comprising: asemiconductor substrate; at least one buffer layer provided over atleast a portion of said substrate; and a plurality of trenchescomprising (a) a plurality of internal trenches that extend into saidsemiconductor substrate and (b) at least one shallow peripheral trenchthat extends into said at least one buffer layer but does not extendinto the semiconductor substrate.
 8. A modified semiconductor substratecomprising a semiconductor substrate; at least one buffer layer providedover at least a portion of said substrate; and a plurality of trenchescomprising (a) a plurality of internal trenches that extend into saidsemiconductor substrate and (b) at least one shallow peripheral trenchthat extends into said at least one buffer layer but does not extendinto the semiconductor substrate, wherein said at least one buffer layeris provided over said semiconductor substrate in the area of said atleast one shallow peripheral trench, wherein said at least one bufferlayer is provided over said semiconductor substrate in the area of saidplurality of internal trenches, wherein said at least one buffer layerin the area of said plurality of internal trenches is thinner than saidat least one buffer layer in the area of said at least one shallowperipheral trench, wherein each internal trench extends through said atleast one buffer layer in the area of said plurality of internaltrenches and into said semiconductor substrate, and wherein each shallowperipheral trench does not extend through said at least one buffer layerin the area of said at least one shallow peripheral trench and does notextend into said semiconductor substrate.
 9. The modified semiconductorsubstrate of claim 8, wherein said at least one buffer layer in the areaof said plurality of internal trenches consists of a single buffer layerand wherein said at least one buffer layer in the area of said at leastone shallow peripheral trench consists of two buffer layers.
 10. Themodified semiconductor substrate of claim 8, wherein the at least onebuffer layer is an oxide layer.
 11. The modified semiconductor substrateof claim 8, wherein the semiconductor substrate is a silicon substrate.12. A trench DMOS transistor comprising the modified substrate of claim8.
 13. A DRAM device comprising the modified substrate of claim 8.